Heat coach Erik Spoelstra might want to figure out how to practice third quarters at some point during this five-game road trip.

The Heat led the Portland Trail Blazers 48-43 at halftime on Thursday night at Moda Center, but then, in familiar form, Miami completely fell apart during the third quarter. The Trail Blazers outscored the Heat 33-16 in the period and Portland went on to hand the Heat a 99-83 loss to begin what looks to be a tough stretch of games for the already struggling defending Eastern Conference champions.

In a promising continuation of its victory against the Brooklyn Nets on Sunday, the Heat held the Trail Blazers to 38 percent shooting in the first half. The Trail Blazers then started the third quarter with a 17-3 run and outscored the Heat 56-35 in the second half.

The Heat (15-21) is last in the league in third-quarter scoring (20.9 points).

“If we had an answer for that there would probably be a different result,” Spoelstra said. “It’s just simply not enough. It’s not enough and the pain of all these frustrating third quarters in games has not ignited the action that is necessary to overcome it. We need a breakthrough in those moments.

“Tonight it was simply a matter of not playing hard enough in those minutes when they made their run.”

Dwyane Wade led the Heat with 23 points, but couldn’t do much against the Trail Blazers’ third-quarter onslaught. He then later missed the backboard on a shot attempt in the fourth quarter and also dribbled the ball off his foot before checking out of the game for good with his team down 21 points. Afterward, Wade said the Heat’s repetitive third-quarter collapses reminded him of romantic comedy movies.

“You know the outcome,” Wade said. “That’s what it felt like. Just typical. I’ve seen this movie before.”

But romantic comedies usually have happy endings. The Heat is now 6-14 since the start of December.

Chris Bosh, who finished with 18 points, was clearly furious after the game.

“I think it’s to the point where it’s just in our minds now,” Bosh said. “It’s a mind thing because it’s not one thing. It’s a culmination of different things over the course of a season and I think it’s just messing with our heads now, and we’re letting it get in there and agitate us, and we end up in these same positions because our brain isn’t right.”

The poor third quarter spoiled an excellent defensive effort in the first half. The Heat led 23-18 after the first quarter and the team got another solid effort off the bench from reserve center Hassan Whiteside. Whiteside had six points, four rebounds and a blocked shot in the second period and finished with 10 points, eight rebounds and four blocks in less than 19 minutes.

The Heat held the Trail Blazers to two three-pointers in the first half, but Portland’s sharpshooters adjusted in the second half. Led by Wesley Matthews, the Trail Blazers went 6 of 13 from behind the arc in the second half. Matthews was 3 of 3 from distance through the third and fourth quarters and finished the game with 18 points.

“I think we got to our tempo,” Matthews said. “Through the first half, we kind of played at their pace, almost methodical, we weren’t really pushing as much.”

The Heat couldn’t keep pace. Miami was 3 of 14 from three-point range for the game. Luol Deng, who was 1 of 3 from three-point range, had seven points in more than 35 minutes on the day after being linked to trade rumors with the Memphis Grizzlies. Deng said before the game that he was happy in Miami and did not want to be traded.

The Heat shot 37.7 percent for the game. Bosh was 5 of 16 from the field and guards Mario Chalmers and Norris Cole combined to go 5-of-12 shooting.

LaMarcus Aldridge led the Trail Blazers with 24 points and 12 rebounds, including six offensive rebounds. Lillard had 16 points on 17 shots. Portland overcame a poor night shooting (42.1 percent overall) by committing just seven turnovers, a season low. The Heat led by 10 points in the first half before giving up the lead.